Radio frequency antenna



y 1957 P. c. MAYBURY ET AL RADIO FREQUENCY ANTENNA 2 Sheefs-Sheet 1Filed July 22, 1953 INVENTORS 7 PAUL O. MAYBURY GILBERT WILKES I WRNEYS1957 P. c. MAYBURY ET AL RADIO FREQUENCY ANTENNA 2 Sheets-Sheet 2 FiledJuly 22, 1953 INVENTORS PAUL G. MAYBURY GILBERT WILKES ATTORNEYS UnitedStates Patent RADIO FREQUENCY ANTENNA Paul C. Maybury, Pikesville, Md.,and Gilbert Wilkes,

Detroit, M1ch., assignors to the United States of Amertea as representedby the Secretary of the Navy Application July 22, 1953, Serial No.369,647

4 Claims. (Cl. 343-753) This invention relates generally to devices forradio frequency wave refraction and more particularly to an improvedantenna for focusing radio frequency (R. F.) energy emitting from .aradar horn or the like on a desired distant restricted area.

The conventional antenna or radiator, such as the electromagnetic horn,the dipole and the waveguide aperture, transmits energy as a divergingwave front, resulting in a low-energy distribution at a distance fromthe radiating element. In examining objects or phenomena in a restrictedregion, the low-energy distribution of such a diverging frontcomplicates a determination of the direct effect of the objects orphenomena on the radiations. Moreover, the incidence of reflectedenergy, because of the divergent character of the wave front, will alterthe effect caused by the objects or phenomena in the region underexamination, making a determination of such effect impossible or atleast unreliable.

In studying the attenuation, reflection and phase effects of the exhauststream of a reaction engine on the transmission of R. F. energy using aconventional radiator, for example, it had been impossible to assign adirect relationship between the measured effect on the transmittedenergy and the effect caused by the exhaust stream. That is, the energypassing through the exhaust stream constituted but a small proportion ofthe total energy transmitted from the radiator. A disturbance of theenergy passing through the exhaust stream, being intrinsically small,therefore affected only a small fraction of the total energytransmitted.

In addition, the divergent wave front produced by the conventional radiofrequency (R. F.) energy radiators used for this purpose gave rise toreflected energy from nearby equipment and structures. This reflectedenergy combined with the transmitted energy passing through thepropellant flame to further complicate and render unreliable thedetermination of the exhaust stream effects.

Devices for focusing microwave radiations have been constructedheretofore. However, because of their complex structure and the closestructural tolerances involved, the successful manufacture of thesedevices was extremely difficult. In certain instances the machininginvolved in manufacturing the device was prohibitive by virtue of thecharacter and size of the material used. In other cases a castingprocess was inappropriate in the interests of homogeneity in the finalproduct, and the final assembly of other devices to the close tolerancesrequired was diflicult. In addition, such prior devices were inflexiblein application, each being designed with a single, unchangeable focallength.

The principal object of the present invention, therefore, is to providean improved antenna principally constituted by a dielectric lens, foruse with a R. F. energy source to converge the R. F. energy radiationsemitting from said source on a distant restricted area.

Another object of this invention is to provide ,a dielectric lens forconverging R. F. energy radiations on a "ice distant restricted area,said lens having a focal length which may be easily altered.

A further object of this invention is to provide a dielectric lens forrefracting R. F. energy radiations which is easily constructed andruggedly built.

Further objects and attendant advantages of the present invention willbecome evident from the following detailed description, taken inconjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view illustrating the method for deriving thegeometry of the dielectric lens constituting this invention;

Fig. 2 is an elevation showing the profile of a dielectric lensconstructed in accordance with this invention;

Fig. 3 is an exploded perspective view of the lens shown in Fig. 2 andan electromagnetic horn with which said lens may be used;

Fig. 4 is an elevation of the lens shown in Fig. 2 mounted on anelectromagnetic horn, and showing the lens and its housing partiallybroken away; and

Fig. 5 is a section on line 5-5 of Fig. 4.

The lens geometry is established under the assumption that R. F. energyradiations emitting from a radiation source obey the principles ofordinary ray optics. Referring to Fig. 1, there is shown a radiationsource S, which may be the waveguide inlet to an electromagnetic horn, adipole, or a waveguide aperture; a lens L, the geometry of which isunknown at this stage, having a base B confronting the source S, and afocus point P at which it is desired to converge the radiations emittingfrom the source S. An equation relating the times of travel ofradiations in the several directions from the source S can be derivedusing the symbols defined as follows with reference to Fig. 1:

a=the distance from the source S to the base B of the lens L;

f=the distance from the base B to the focus point P;

y=the radius of the base B;

x=a variable distance from the base B along the axis of the lens L andconstitutes the abscissa of the lens profile;

y=the radial dimension of the lens L at the distance x from the base Band constitutes the ordinate of the lens profile.

The paths of the individual radiations through lens L are considered tobe parallel to the axis of said lens in the interests of simplicity Thisassumption is reasonable if the distance a from the source S to the baseB is large compared with the diameter of said base, inasmuch as theindividual radiations will then enter the lens L substantially normal tothe base thereof and little refraction will occur.

The complete distance of travel of each radiation from the source S tothe focus point P is represented by the summation of the followingdistances which may be expressed in terms of the variables x and y bythe application of the Pythagoran principle.

Where z=distance of path from the source S to the base B;

l=distance of path through the lens L;

d=distance of path from the lens L to the focus point P; and

n=the index of refraction of the lens material, then dam-ma 3 The timeof travel of each ray from the source S to the focus point F, therefore,is

where c=speed of the rays which is constant.

' If the radiations converge on the focus point P, then the time oftravel of every ray from the source S to the focus point F is the same.Thus, imposing a time cou The time constant K may be determined from theEquation 1 by inserting therein values corresponding to a radiationpassing through the outermost peripheral portion of the lens L.

In order to calculate the lens geometry, it is necessary to specify theparticular circumstances'under which the lens is intended to operate.More specifically, the values of a, f, y and It must be fixed. The valueof a and y may be limited by the structure of the radiation source, i.e. if the source comprises an electromagnetic horn the value of a is theaxial length of said horn, and y is the radius of the outlet of saidhorn. Also, the values of a and y may be determined as a matter ofconvenience when there are no structural limitations. The value of f, ofcourse, depends upon the focal length desired, and the value of n isgoverned by the chosen lens material. A substitution of the specifiedvalues into Equation 1 provides a mathematical expression representativeof a parabola which defines the lens profile.

In constructing a lens in accordance with this invention, the parabolicprofile is approximated. As shown in Figs. 2, 3, 4 and 5, a lens L isconstructed of a series of disks 11 of varying diameters held in stackedrelation to form a generally stepped paraboloidal solid 12 having a base13 and an apex 14. The disks 11, from the base 13 to the apex 14, are ofsuccessively decreasing diameters so that an imaginary plane passingthrough the axis of the solid 12 intersects the peripheries of the rearfaces of each of said disks to form a series of points the locii ofwhich describe a parabola corresponding to the calculated lens profile.A hole 15 is concentrically formed in each of the disks 11 and a dowel16 extends halfway into the holes of adjacent disks to retain said disksin position. This type of construction permits certain of the disks 11to be interchanged with other disks in order to vary the focal length ofthe lens L or to replace damaged disks.

In mounting the lens described above on an electromagnetic energyradiating horn, reference is made to Figs. 3, 4 and 5. A horn 17 isshown in these figures as comprising a conically shaped portion 18formed with a waveguide terminal 19 at its convergent end, and acircular mouth 21 and a flange 22 at its divergent end. The flange 22 isprovided with a series of equally spaced holes 23 therein and is adaptedto mount a lens assembly including a dielectric lens L and a housing 24,in a manner to be described hereinafter.

A clamping band 25 having its end portions joined by a bolt and nut 26fits around the base portion of the lens L so that by tightening saidnut on said bolt the lens base portion is gripped by said band. Aplurality of brackets 27, corresponding in number to the number of holes23 in the flange 22 of the horn, are mounted in spaced relation on theperiphery of the band 25. Each of the brackets 27 is constructed with abase leg 28 having an arcuate slot 29 therein and a perpendicularlyextending leg 31 for connection to the band 25, as by rivets 32. Thebase leg 28 of each bracket is adapted to be secured to the flange 22 ofthe horn 17, as will appear hereinafter.

The housing 24 comprises a cylindrical shell 33 having an inwardlydirected flange 34 on one end and a cylindrical can 35- slidably fittingover the other end. The flange 34 is formed with a series of holes 36corresponding to those in the flange 34 of the horn 17, for mountingpurposes as will be described hereinafter. The lens L, with the clampingband 25 gripping its base portion, is contained within the shell 33 andthe base legs 28 of the brackets 27 abut the inner face of the flange 34of the shell 33.

The can 35 is provided with an inwardly directed flange 37 at its outerend. A circular plate 38 of dielectric material fits within the can 35and abuts the inside face of the flange 37 and closes the outer end ofsaid can. Nuts 39 cooperate with bolts 41, passing through suitableholes 42 formed in the flange 37 and the plate 38, to hold said plate tosaid flange. Four equally spaced longitudinal slots 43 are formed in theinner end portion of the can 35 and receive four bolts 44 mounted on theshell 33. Wing nuts 45 cooperate with thev bolts 44 to adjustably fastenthe shell and the can together. In fitting the can 35 over the shell 33the circular plate 38 is caused to abut the apex 14 of the lens L toprovide further support therefor.

A plurality of bolts 46 pass through the holes 23 in the flange 22 ofthe horn 17, the corresponding holes 36 in the flange 34 on the shell33, and are received by corresponding slots 29 in the base plates 28 ofthe brackets 27. A washer- 47 is placed on each of the bolts 46 and anut 48 is drawn up on each of said bolts to secure the housing24 andthelens L to the horn 17.

The lens L without the housing 24 provides a definite focusing of R. F.waves in a restricted region. However, a slight phase error occurs atthe outer edge of the lens L. The'primary purpose of the housing is toprotect the lens from mechanical damage. However, the flange 37 of thecan 35 of said housing functions as an inductive window which can besuitably located with respect to the lens to compensate for the slightphase errors encountered. around the outer edge of the lens L.

The lens L with its housing 24 in place is further characterized by asmall focal depth. Even in spite of a rapidly decreasing range, whichnormally operates to increase the transmitted power inversely to thesquare of the range, the. transmitted power actually rapidly decreases.For an increasing range, the transmitted power similarly decreasesrapidly.

The characteristics of the lens of the present invention are more fullyunderstood with reference to a lens of particular dimensions. A lensprofile is derived for the following conditions:

a=1.625 feet y'=0.5 feet i=4 feet 71:1.1

The lens material chosen is a substance commonly known as Celltite andis a foam material produced by the Sponge Rubber Company of Shelton,Connecticut. Celltite has a refractive index of approximately 1.1 and alow loss tangent of approximately 0.0017, thus assuring a reasonablevolume. The combination of the moderate refractive index and the lowloss tangent of this substance results in a low attenuation of energypassing through the lens. In addition, the moderate refractive indexpermits the simplifying approximation of the lens profile.

A lens constructed in accordance with the limitations set forth above,when mounted on an electromagnetic energy radiating horn, provides anenergy distribution in the focal plane (four feet from the lens base)having half-power points less than four inches apart. This separation ofhalf-power points corresponds to a subtended angle of approximately fivedegrees which is similar in order of magnitude to the angle subtended bythe halfpower points of the horn without the lens in the distant zone.Thus, the lens may be considered as modifying the wavefront of wavesemitting from the horn so that distant zone patterns of energydistribution are obtained even in the near Zone.

Optimum energy concentration is obtained with this lens at the focalplane, a distance of four feet from the base of the lens. The energyconcentration progressively and rapidly decreases from the focal planedirectly towards and away from the lens, thereby indicating an energyconcentration of small depth at the focal plane.

A large straight-edged metal plate coincident with the focal plane butat a distance from the lens axis does not effect the transmitted energyuntil the leading edge of the plate is advanced to within three inchesfrom said axis. As the plate is advanced further towards the lens axisthe transmitted energy rapidly declines until a nearly zero value isreached when the leading edge of said plate extends approximately threeinches beyond the lens axis. A concentration of energy is thus definedto be within a three inch radius about the lens axis in the focal plane.

Because of the sharp focusing of R. F. wave energy brought about throughthe use of lenses constructed in accordance with this invention, it ispossible to investigate the effects of objects and phenomena inrestricted areas. The confusing influences of reflections from adjacentstructures and the ground are effectively eliminated, for all practicalpurposes, thus permitting accurate indoor investigations to beperformed.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. it is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. An R. F. energy antenna for focusing R. F. energy radiations on adistant restricted area, comprising a source for emitting R. F. energyradiations and a dielectric lens having an axis passing through saidsource, said lens comprising a series of homogeneous disks in coaxialstacked relation, said disks being of different diameters so as togenerally form a paraboloidal solid having a base confronting saidsource and an axial cross-section defined by a curve represented by thefollowing equation wherein a is the distance from the source to the baseof said solid, f is the distance from said base to the distant focalarea, It is the refractive index of said disks, x is a distance fromsaid base along the axis of said solid, y is the radial dimension of thesolid at the distance x and K is represented by K=va r +vf r where y isthe radius of the base of said solid, said radius y being substantiallyless than said distance a.

2. A dielectric lens for focusing R. F. energy radiations emitting froma substantially point source on a distant restricted area, comprising aseries of homogeneous disks in stacked relation, said disks being ofdifierent p K? diameters so as to generally form a paraboloidal solidhaving a base and an axial cross-section defined by a curve representedby the following equation +y +x (f +y wherein a is the distance from thepoint source to the base of said solid, 1 is the distance from the baseof said solid to the distant focal area, n is the refractive index ofsaid disks, x is a distance from said base along the axis of the solid,32 is the radial dimension of the solid at the distance x and K isrepresented by =w/ +(y') f +(y) where y is the radius of the base ofsaid solid, said radius y being substantially less than said distance a.

3. A dielectric lens for focusing R. F. energy radiations emitting froma substantialy point source on a distant restricted area, comprising anadjustable cylindrical housing open at two ends, a dielectric plateclosing one end of said housing, and a series of homogeneous dielectricdisks in coaxial stacked relation mounted within said housing andclosing the other end of said housing, said disks being of differentdameters so as to generally form a paraboloidal solid having a base andan axial cross-section defined by a curve represented by the followingequation =1/ +y +\/(f +y wherein a is the distance from the point sourceto the base of said solid, is the distance from the base of said solidto the distant focal area, n is the refractive index of said disks, x isa distance from said base along the axis of the solid, y is the radialdistance of the solid at the distance x and K is represented by K= /a+(y) /f +(y) where y is the radius of the base of the solid, said radiusy being substantially less than said distance a.

4. An arrangement as set forth in claim 3 wherein, the housing comprisesa cylindrical shell, means for supporting the lens in one end of saidshell, a cylindrical can slidable on said shell to adjust the axiallength of said housing to accommodate different size lenses, adielectric plate closing the outer end of said can and abutting againstthe outermost disk of said lens, and an inwardly directed flange on theouter end of said can for minimizing phase error.

References Cited in the file of this patent UNITED STATES PATENTS625,823 Zickler May 30, 1899 2,202,380 Hollmann May 28, 1940 2,547,416Skellett Apr. 3, 1951 2,577,619 Kock Dec. 4, 1951 OTHER REFERENCESArticle by A. H. Lince, FM-TV, vol. 12, issue 3, March 1952, pages22-24.

